A step unit is arranged adjacent to a sliding door in a vehicle body. For example, with reference to NPL 1, the step unit includes a step member and a rail plate member. A lower rail extends in the direction in which a sliding door opens and closes on a lower surface of the step member. The lower rail supports rollers coupled to the sliding door. Thus, the rollers and the sliding door are guided along the lower rail. A cutout portion is formed in part of the lower rail. The rail plate member is coupled to the lower rail in a removable manner at the cutout portion. When the rail plate member is removed from the lower rail, the rollers may be supported by the lower rail or be removed from the lower rail through the cutout portion.